Since Fawcett disclosed the possibility of polymerizing olefins, there have been many significant improvements in olefin polymers and methods for obtaining them.
A milestone was the introduction by Ziegler of a transition metal catalyst which would produce high molecular weight solid polymers at low pressures. Other low pressure catalysts have since emerged each providing advantages and at times disadvantages over the catalyst system proposed by Ziegler.
The advent of the low pressure catayst systems focused new attention on processes for polymerizing olefins. The most significant commercial process to date is the slurry process wherein a polymer is prepared in a solvent under constant stirring in the presence of a low pressure catalyst. The most inherent deficiency in the slurry process is the solvent which must be recovered at some operating cost and at the expense of solvent depletion. Some solvent may also remain in the polymer thereby affecting the physical properties of product. The physical properties of the slurry also complicates this deficiency and most slurry reactors are operated at a high solvent content to insure that an agitatable mass will be maintained.
It must be appreciated therefore that a solvent free, low pressure reaction system would offer advantages over the slurry process.
U.S. Pat. No. 3,023,203 proposes such a system. It proposes a process wherein an olefin is polymerized in a three-zone vapor phase reactor comprising a lower product collection zone from which a self-deposited polymer product is extruded without a pressure drop, an intermediate reaction zone and an upper knock-out zone for returning "fines" to the reaction zone. The fluidizable catalyst system comprises a hexavalent chromium-oxide catalyst disclosed in U.S. Pat. No. 2,825,721 deposited on a carrier having a particle size of up to 40 mesh. According to the process a polymer is allowed to grow on a catalyst particle until it becomes large enough to drop from the reaction zone into the product collection zone.
A difficulty with such a process is that the particles which settle in the product collection zone may still be active and growing. Since heat transfer is low in such a quiescent mass, the generated heat of reaction there is likely to cause fusion of particles thereby plugging the product collection zone.
A vapor-phase polymerization process has also been proposed in British patent specification No. 910,261, wherein a Ziegler catalyst is shown to cause the formation of olefin polymers in a fluidized bed reactor. The retention of catalyst by the polymer product, however, imposes a limitation on this process. Since the Ziegler catalyst is corrosive to molds and the like, the catalyst residue must be removed from the polymer by solution and washing thereby obviating much of the advantage provided by vapor-phase polymerization.
The problems outlined above have heretofore substantially prevented the commercial exploration of low-pressure vapor-phase reactors for the polymerization of olefins.